INTEGRATING DISCHARGE AND RAINFALL INDICATORS IN RIVER AND FLOODPLAIN DEPOSITS: AN EXAMPLE SPANNING THE PALEOCENE-EOCENE THERMAL MAXIMUM

The Paleocene-Eocene Thermal Maximum (PETM; ca. 56 Ma) is one of the best-constrained examples of rapid climate change in the geologic record. The event is linked to a massive release of exogenic carbon into Earth's atmosphere and oceans over a timescale of tens of thousands of years or less, with elevated carbon dioxide levels persisting for at least another one hundred thousand years. Both models and field data constrain global temperature increases and changes to the hydrologic cycle. While mean annual temperature and precipitation are important climatic parameters, seasonality and interannual variability within the climate system remain largely unconstrained. Subtle features of both river and floodplain deposits may hold important information about these climate parameters. Herein we present a comparison of detailed lithofacies analyses for contemporaneous river and floodplain deposits spanning the PETM in the Piceance and Bighorn basins. Climate models predict increases in seasonality in both locations and the physical characteristics of the deposits broadly support this hypothesis. However, the specifics are drastically different between the two basins. The Piceance Basin exhibits changes in channel-stacking, grain-size, channel dimensions, and the prevalence of upper flow regime structures. In contrast, the Bighorn Basin only shows changes in channel-stacking during the PETM. Floodplain deposits in both basins show changes in pedogenic mottling, nodule development and structures consistent with variations in rainfall. Comparison using a semi-quantitative soil morphology index reveals greater variability in the Piceance Basin compared to published records in the Bighorn Basin. We tentatively interpret that the Piceance Basin experienced a greater amount of variation in rainfall compared to the Bighorn Basin before, during, and after the PETM. However, there are several caveats including time averaging of several hundreds to thousands of years per soil examined, unclear relationships between channel-forming discharge and rainfall variability, and likely lags and hysteresis within the geomorphic system. Despite overall consistency with the climatic changes, direct linear interpretations of the stratigraphic record as a measure of climatic variability should be done with caution.